A DC-DC converter is an electronic circuit that converts a source of direct current (DC) from one voltage to another. For example, DC-DC converters are widely used in portable devices to provide power from a battery. DC-DC converters may also regulate the output voltage, compensating for varying load current and variations in the input voltage.
FIG. 1 illustrates one common type of DC-DC converter. The DC-DC converter circuit 100 in FIG. 1 (simplified to facilitate illustration and description) is a switching step-down converter (the input voltage is higher than the output voltage), and the basic design is called a Buck converter. In FIG. 1, a power source 102 provides direct current at an input voltage VIN. The circuit 100 provides direct current to a load (RLOAD) at an output voltage VOUT. Two electronic switches (SW1, SW2) are controlled by a switch control circuit 106 and driver 108. At most only one switch is closed at any one time. When SW1 is closed, current flows into RLOAD and a filter capacitor (COUT) from the source 102, and VOUT rises linearly. In addition, when SW1 is closed, energy is stored in LO and COUT. When SW2 is closed, current flows from stored energy in COUT and from stored energy in LO, and VOUT decreases linearly. A comparator 104 compares VOUT to a reference voltage VREF, and the switch control circuit 106 adjusts the closing frequency or duty cycle of SW1 in response to the output of comparator 104.
There are many variations in topology and control of DC-DC converters. The circuit illustrated in FIG. 1 uses output voltage feedback. Some circuits use current feedback. Some circuits have multiple feedback loops. In general, there are advantages and disadvantages of each variation, and some applications have special requirements. In particular, power supplies for advanced digital circuits have a challenging set of requirements. Microprocessor cores, digital signal processors, and other devices may switch rapidly from sleep-mode to full-power and full-power back to sleep-mode, requiring a fast response by the power supply to sudden load changes. In addition, such electronic devices have strict limits on the peak-peak transient output voltage change that is allowed during the response to a load transient. In addition, power supplies for advanced digital circuits need to accommodate a wide range of input voltages and output voltages, need to provide power at low output voltages (for example, 1V), and need to be robust to electronic system noise, particularly ground noise.
FIG. 2 illustrates an example DC-DC converter 200 based on a step-down switching circuit as in FIG. 1, with enhancements that are particularly suitable for meeting many of the general goals for supplying power to advanced digital circuits. Circuit 200 in FIG. 2 (simplified to facilitate illustration and discussion) is called a DE-DRC (Differentially Enhanced-Duty Ripple Control) circuit. The DE-DRC design has been previously published by the present inventor and others. See, for example, J. Fan, X. Li, S. Lim, and A. Huang, “Design and Characterization of Differentially Enhanced Duty Ripple Control (DE-DRC) for Step-Down Converter,” IEEE Trans. Power Electron., vol. 24, no. 12, pp 2714-2725, December 2009. In FIG. 2, a power source 202 provides direct current at an input voltage VIN.
The circuit 200 provides direct current to a load (RLOAD) at an output voltage VOUT. Two electronic switches (SW1, SW2) are controlled by a switch control circuit 210 and driver 212. At most only one switch is closed at any one time. A comparator 208 controls the switch control circuit 210. There are two feedback paths. In a first feedback path, the voltage VSW on the switched side of the inductor LO is coupled to the comparator 208 through a low pass filter (RR, CR). In a second feedback path, two differential difference amplifiers (204, 206) generate a differential pair of feedback signals, VP and VN. These two differential signals are coupled to comparator 208, where VN is directly coupled, and VP is coupled through a high pass filter (CR, RR).
VP and VN are as follows:VP=KP(HVOUT−VREF)+VOUT VN=−KN(HVOUT−VREF)+VOUT     Where H=RS2/(RS1+RS2), KP=gain of positive differential circuit 204, and KN=gain of negative differential circuit 206, and VREF is a constant reference voltage.
The first feedback path coupling VSW to the comparator 208 is fast because there are only passive components between VSW and the comparator. The feedback path coupling VOUT to the comparator 208 is slower, because VOUT is proportional to the integral of current in COUT, and there are active amplifiers between VOUT and the comparator. The second loop can adjust the control bandwidth. Both loops have a big influence on the transient response. The two feedback loops combined provide a stable system with high bandwidth control.
To simplify the discussion, assume H=1 and VIN is constant. First, consider the steady state (constant load). In the steady state, with H=1, VCONTROL (the positive input of the comparator 208) is approximately VOUT. VSW is a square wave, having an average value of approximately VOUT, and low-pass filtered VSW contributes a sawtooth waveform having an average value of approximately the average value of VSW to VRIPPLE (the negative input of the comparator 208). When VRIPPLE drops below VCONTROL, comparator 208 causes switch control circuit 210 to close SW1 for a constant on-time. VSW is driven higher while SW1 is closed, and VOUT is controlled to be approximately equal to the average value of VSW. If the load current increases, the average value of VSW increases rapidly (because of increasing current through LO), VP has a transient decrease (VOUT decreases due to an increased current draw from COUT), VN has a transient increase, and the duty cycle of SW1 is increased (switching frequency increases). If the load current decreases, the average value of VSW rapidly decreases, VP has a transient increase, VN has a transient decrease, and the duty cycle of SW1 is decreased. The magnitude of VRIPPLE at the negative input of the comparator 208 is relatively large compared to the ripple voltage on VOUT, which provides good noise immunity in the feedback signal. In addition, the circuit provides a fast response to load transients over a wide input and output range.
Even though the DE-DRC circuit of FIG. 2 is particularly suitable for providing power to advanced digital systems, there is an ongoing need for further improvements in efficiency, noise immunity, output accuracy, output range, and so forth, for DC-DC converters.